Synthesis of load-independent ac drive systems

Abstract

A method of synthesizing load invariant ac synchronous and asynchronous motor drive systems comprising positive stator current feedback of exactly specified nature and value of its transfer functions. The system transfer function independent of load is realized while stability and dynamics of the system are controlled by additional voltage loop.

We claim: 
     
       1. A method for synthesizing load independent alternating current drive system comprising: accepting a source of electrical energy of a constant voltage at an input,   coupling mechanically an alternating current synchronous motor shaft to a load to be drive at an output,   controlling a power flow from said input to said output, modulating a power converter for the control of said power flow in a pulse width modulation control manner, supplying a total control signal for modulating said power converter,   supplying position feedback pulses,   feeding back side position feedback pulses in a negative feedback loop with respect to a position command pulses and comparing frequency and phase of the two pulse trains in a phase frequency detector; thereby producing a position error voltage signal proportional to a difference in frequency and phase between the two pulse trains, supplying a velocity feedback signal,   feeding back said velocity feedback signal in a negative feedback loop with respect to a velocity command voltage and the position error signal and summing the three voltages, passing a signal obtained as the algebraic sum of the velocity feedback signal and the velocity command signal and the position error signal through a cascas connection of a filtering and stabilizing network and a control circuit; thereby producing a control signal proportional to the algebraic sum of the velocity command signal and the velocity feedback signal and the position error signal,   sensing a current through an alternating current synchronous motor stator,   feeding back the sensed current signal through a current feedback circuit in a positive feedback loop with respect to said control signal and summing the two signals,   supplying said total control signal, obtained as the sum of said control signal and the current signal fed through said current feedback circuit, for modulating said power converter for the control of the flow of power from the input electrical source to the output mechanical load, whereby the alternating current synchronous motor shaft position and velocity is made independent of said load.   
     
     
       2. The method of claim 1 wherein said current feedback circuit in said positive feedback loop is synthesized using an equation providing transfer function in laplace domain of said current feedback circuit   H(s)=[R.sub.st +sL.sub.st ]/[R(R.sub.4 /R.sub.3)A]     in said equation R st  being a stator resistance per phase, L st  being a synchronous inductance per phase consisting of a sum of a stator leakage inductance L sl  and a magnetizing inductance L m , R being a transresistance of a motor current sense device, R 4  /R 3  being a gain of a buffering differential amplifier in a motor current sensing circuit, A being a voltage gain of a pulse width modulation control and power stage, and s being a complex frequency laplace variable in the transfer function H(s).   
     
     
       3. The method of claim 2 wherein said equation providing transfer function of said current feedback circuit is physically implemented, thereby implementing said current feedback circuit, as a differentiator circuit with a direct current path wherein said differentiator circuit with said direct current path is arranged to provide a differentiating time constant equal to   L.sub.st /[R(R.sub.4 /R.sub.3)A]     and a direct current gain constant equal to     R.sub.st /[R(R.sub.4 /R.sub.3)A],     L st  being the synchronous inductance per phase consisting of the sum of the stator leakage inductance L sl  and the magnetizing inductance L m , R being the motor current sense device transresistance, R 4  /R 3  being the gain of the buffering differential amplifier in the motor current sensing circuit, A being the overall voltage gain of the pulse width modulation stage, and R st  being the stator resistance per phase.   
     
     
       4. A method for synthesizing load independent alternating current drive system comprising: accepting a source of electrical energy of a constant voltage at an input,   coupling mechanically an alternating current asynchronous motor shaft to a load to be driven at an output,   controlling a power flow from said input to said output, modulating a power converter for the control of said power flow in a pulse width modulation control manner, supplying a total control signal for modulating said power converter,   supplying position feedback pulses,   feeding back said position feedback pulses in a negative feedback loop with respect to a position command pulses and comparing frequency and phase of the two pulse trains in a phase frequency detector; thereby producing a position error voltage signal proportional to a difference in frequency and phase between the two pulse trains,   supplying a velocity feedback signal,   feeding back said velocity feedback signal in a negative feedback loop with respect to a velocity command voltage and the position error signal and summing the three voltages, passing a signal obtained a the algebraic sum of the velocity feedback signal and the velocity command signal and the position error signal through a cascade connection of a filtering and stabilizing network and a control circuit; thereby producing a control signal proportional to the algebraic sum of the velocity command signal and the velocity feedback signal and the position error signal,   sensing a current through an alternating current asynchronous motor stator,   feeding back the sensed current signal through a current feedback circuit in a positive feedback loop with respect to said control signal and summing the two signals,   supplying said total control signal, obtained as the sum of said control signal and the current signal fed through said current feedback circuit, for modulating said power converter for the control of the flow of power from the input electrical source to the output mechanical load, wehreby the alternating current asynchronous motor shaft postion and velocity is made independent of said load.   
     
     
       5. The method of claim 4 wherein said current feedback circuit in said positive feedback loop is synthesized using an equation providing transfer function in laplace domain of said current feedback circuit   H(s)=[R.sub.st +sL.sub.st ]/[R(R.sub.4 /R.sub.3)A]+[a.sub.1 s.sup.2 +a.sub.2 s]/[a.sub.3 s+1]     in said equation R st  being a stator resistance per phase, L st  being a stator leakage inductance per phase, R being a transresistance of a motor current sense device, R 4  /R 3  being a gain of a buffering differential amplifier in a motor current sensing circuit, A being a voltage gain of a pulse width modulation control and power stage, a 1  being a parameter obtained as     a.sub.1 =[L.sub.m L.sub.rs ]/[R(R.sub.4 /R.sub.3) AR.sub.rs],     L m  being a magnetizing inductance per phase, L rs  being a rotor leakage inductance referred to stator per phase, R rs  being a rotor fictitious resistance referred to stator per phase obtained as     R.sub.rs =R.sub.rs.sup.' /s.sub.l,     R rs   '  being a rotor resistance referred to stator per phase, s l  being a slip of an asynchronous motor,   a 2  being a parameter obtained as   a.sub.2 =[L.sub.m ]/[R(R.sub.4 /R.sub.3)A],       a 3  being a parameter obtained as   a.sub.3 =[L.sub.m +L.sub.rs ]/R.sub.rs,      and s being a complex frequency laplace variable in the transfer function H(s).     
     
     
       6. The method of claim 5 wherein said equation providing transfer function of said current feedback circuit is physically implemented, thereby implementing said current feedback circuit, as a parallel connection of a two signal processing circuits, the first of th two circuits processing a stator effects and physically implemented as a differentiator circuit with a direct current path wherein said differentiator circuit with said direct current path is arranged to provide a differentiating time constant equal to   L.sub.st /[R(R.sub.4 /R.sub.3)A]     and a direct current gain constant equal to     R.sub.st /[R(R.sub.4 /R.sub.3)A],     and the second of the two circuits processing a rotor effects and physically implemented as a cascade connection of a two circuits the first one of which being a parallel connection of a double differentiator circuit of a differentiating time constant of each of the two differentiators in said double differentiator circuit equal to ##EQU2## with a differentiator circuit of a differentiating time constant equal to     [L.sub.m ]/[R(R.sub.4 /R.sub.3)A]     and the second one of which being an integrator with a direct current path wherein said integrator with said direct current path is arranged to provide an integration time constant equal to     [L.sub.m +L.sub.rs ]/R.sub.rs     and a direct current gain constant equal to one, whereby said parallel connection of said two signal processing circuits and said parallel connection of said double differentiator circuit and said differentiator circuit are each implemented summing the output as of the respective circuits in the respective parallel connections in a summing circuits, in the differentiating time constants and the direct current gain constant and the integration time constant L st  being the stator leakage inductance per phase, R being the motor current senses device transresistance, R 4  /R 3  being the gain of the buffering differential amplifier in the motor current sensing circuit, A being the overall voltage gain of the pulse width modulation stage, R st  being the stator resistance per phase, L m  being the magnetizing inductance per phase, L rs  being the rotor leakage inductance referred to stator per phase, and R rs  being the rotor fictitious resistance referred to stator per phase.